Lipoproteins are one of the conjugated proteins occurring widely in nature and having a structure in which at least one protein and some lipids are conjugated with each other.
Typical examples of the food materials which have been widely used are milk and egg yolk. For example, most of the proteins and lipids in egg yolk are present in the form of a lipoprotein.
The proteins and lipids, both of which constitute a lipoprotein, are indispensable nutrients to humankind. There is, however, a great demand for food having a low lipid content, based on a tendency to intake an excess of lipids, and people have an increasing consciousness that the intake of particular lipid ingredients such as cholesterol should be controlled.
In particular, egg yolk is a kind of complete nutritious food because not only does it have a good taste and a high nutritive value but also it is rich both in various vitamins and iron content. Also, the functions of egg yolk, such as emulsifiability and thermocoagulability, have been utilized in various forms in processed food. However, because egg yolk contains some lipids, particularly cholesterol, at a high concentration, the intake of egg yolk is often controlled not only to treat patients with hyperlipidemia but also to keep a body in good health for normal healthy people. It is, therefore, requested to reduce the lipid content, particularly cholesterol content, in egg yolk without deteriorating its taste and functions.
On the other hand, much attention is now given to blood plasma of slaughtered animals which has not yet been utilized as a food material but would become a useful protein material in the future. Of course, blood plasma of slaughtered animals also contains a great amount of lipoproteins and is is well known that they are associated with the control of lipid transport and the control of intracellular lipid metabolism. The lipids contained in the lipoproteins is the primary causative substance for development of a peculiar odor. Therefore, the blood plasma of slaughtered animals cannot be expected to find applications as a food material until the lipid content therein is reduced.
A general process is known for removing lipids from lipoprotein-containing substances, in which extraction is conducted with an organic solvent such as ethanol, methanol and chloroform, or a mixture thereof. This process is only used as an analytical technique and no attempt has been made to utilize it on an industrial scale.
In addition, as means for removing lipids from egg yolk which is a typical example of the lipoprotein-containing substances, there have been proposed extraction with dimethylether (JP-B 60-9770) and extraction with supercritical carbon dioxide (JP-A 59-135847, 59-140299 and 3-98541), both of which are removal techniques only applied to egg yolk in a dry state. For the purpose of removing lipids, particularly cholesterol, from liquid egg yolk, there have been proposed a few methods utilizing the mixing of egg yolk with edible oil (U.S. Pat Nos. 3,717,474 and 4,333,959, and GB-2238456). Moreover, a process for removal by extraction of cholesterol from liquid egg yolk which is brought into contact with supercritical carbon dioxide is known from U.S. Pat. Nos. 5,116,628 and 5,238,694.
In the case of a process using an organic solvent, many problems will occur, such as a change in emulsifiability and thermocoagulability, caused by the denaturation of proteins; deterioration of flavor and feel of eating; and residual solvent.
As described above, lipoproteins comprise at least one protein and some lipids ingredients conjugated therewith, and they take a stable structure in water; therefore, any technique which uses a raw material in the dry state has disadvantages in that the protein is denatured and the lipid ingredients are oxidized in the step of heating the raw material. Moreover, the structure of the lipoproteins is broken by removal of water, and therefore, functional characteristics, such as emulsifiability, which are inherent to lipoprotein-containing substances, are deteriorated. Further, the drying step requires a great amount of heat energy, which also makes this technique unfavorable from the economical point of view.
In the process for removing cholesterol by mixing liquid egg yolk with edible oil, much force is required for stirring and shearing, and it is necessary to use a great amount of edible oil as an extraction agent. For this reason, such a process finds no industrial application. Moreover, the lipids contained in the lipoprotein-containing substance may be replaced by the edible oil used as an extraction agent in the step of extraction, thereby making it substantially impossible to reduce the lipid content. Further, much force applied for stirring and shearing, as well as heat generated thereby, may deteriorate the functional characteristics of the lipoprotein-containing substance, similarly to the case of a technique using a raw material in a dry state. Thus, there has been much difficulty so far in the removal of lipids from a lipoprotein-containing substance usually in a liquid state without causing many problems as described above, i.e., without making any change in the properties of the substance and causing any deterioration of its functional characteristics.
A process have been found for removing cholesterol from liquid egg yolk under mild conditions, by utilizing the characteristics of a supercritical fluid and by employing a wetted wall column system in bringing the liquid egg yolk into contact with the supercritical fluid. However, this process requires much time for removal of cholesterol by extraction because lipoproteins have a stable structure as described above, and it cannot yet be said that this process is satisfactory for practical use.